This disclosure relates to water treatment devices that utilize photocatalytic oxidation to generate oxidizing agents. A water treatment device is configured to be connected in line between a water supply and a washing machine. Wash water from the water supply passes into the device and through a conduit of the device. A reaction chamber includes an ultraviolet lamp and a catalyst cell for generating the oxidizing agents. The generated oxidizing agents are injected into the wash water as it passes through the conduit. The water treatment device also includes a flow sensor placed in line with the conduit to determine when the device should be activated to generate the oxidizing agents.